11 July, 2025

Why Bioengineer Photosynthesis?


  • Growing global population and climate change threaten food security
    • there is limited land to grow more groups


  • C3 crops (e.g., rice, wheat, soybean) are limited by photorespiration, inefficient carbon fixation, and inefficient water use
    • all made worse under climate change factors (heat and drought)



  • Improving photosynthesis is a promising strategy for increasing crop yield and resilience
    • Bioengineering photosynthesis is a frontier for sustainable agriculture

The Problem with C3 Photosynthesis: Review


  • Rubisco, the key enzyme, can bind O2 instead of CO2, especially under heat and drought
    • leads to photorespiration – a wasteful process that reduces yield
    • forces stomata to be open more = more water loss
    • reduces carbon fixation


  • C3 plants lack mechanisms to concentrate CO2 around Rubisco


  • Eliminating photorespiration as a process is unrealistic
    • but can we tweak Rubisco or other processes?

Why Bioengineer Photosynthesis?




  • Any improvement = huge gains in crop yield


  • 5% reduction in photorespiration =
    • extra 68 million bushels of soybean
    • extra 23 million bushels of wheat
    • $540 million value (Walker et al. 2016)


  • C3 crops can lose 10-40% of yield due to water related stress
    • water is transpired at a rate of hundreds of molecules per one CO2 fixed

Approaches to Photosynthesis Engineering are Complex



1-Photorespiration produces a toxic byproduct called glycolate, which wastes energy to detoxify
Engineeer new metabolic pathways (from bacteria or algae) into the plant’s chloroplasts to quickly recycle glycolate into useful products


2-Rubisco has a higher affinity for O2 than CO2 and is also slow
Modify Rubisco’s structure or assemble Rubisco from cyanobacteria or algae that have better kinetic properties


3-The dilemma of stomatal behavior limits photosynthesis
Introduce PEPC and nocturnal CO2 fixation pathways (via CAM pathway)


4-The messy structure of C3 leaf anatomy is inefficient in supplying light and CO2
Alter regulatory genes for mesophyll density, vein spacing, intercellular air space, or stomatal distribution
Alter regulatory genes that control Kranz anatomy (C4) and express C4 enzymes in the right cell types

Is C4 Photosynthesis the Cure?



  • C4 plants (e.g., maize, sugarcane) spatially separate CO2 fixation and the Calvin cycle


  • CO2 is first fixed in mesophyll cells (via PEPC), then shuttled to bundle sheath cells where the Calvin cycle occurs
    • reduces photorespiration and increases efficiency (CO2 released, O2 kept out)
    • especially in hot, dry environments.


  • C4 plants can have up to 50% higher photosynthetic rates under high light, heat, and drought compared to C3 plants
    • use about half as much water to fix the same amount of carbon

The C4 Rice Project



  • C4 rice is a flagship project, with potential for higher yields and reduced water use


  • Goal: Introduce C4 traits into rice (a C3 crop) to increase photosynthetic efficiency
    • Focus areas: installing Kranz anatomy, expressing C4 enzymes
    • Challenges: complex genetic regulation and developmental changes


  • Understanding photosynthetic anatomy and genetics is key to the next breakthroughs

It is time for a …..


Understanding photosynthetic anatomy and genetics is key to the next breakthroughs.